174 строки
5.3 KiB
ReStructuredText
174 строки
5.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Block Group Descriptors
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-----------------------
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Each block group on the filesystem has one of these descriptors
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associated with it. As noted in the Layout section above, the group
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descriptors (if present) are the second item in the block group. The
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standard configuration is for each block group to contain a full copy of
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the block group descriptor table unless the sparse_super feature flag
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is set.
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Notice how the group descriptor records the location of both bitmaps and
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the inode table (i.e. they can float). This means that within a block
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group, the only data structures with fixed locations are the superblock
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and the group descriptor table. The flex_bg mechanism uses this
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property to group several block groups into a flex group and lay out all
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of the groups' bitmaps and inode tables into one long run in the first
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group of the flex group.
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If the meta_bg feature flag is set, then several block groups are
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grouped together into a meta group. Note that in the meta_bg case,
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however, the first and last two block groups within the larger meta
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group contain only group descriptors for the groups inside the meta
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group.
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flex_bg and meta_bg do not appear to be mutually exclusive features.
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In ext2, ext3, and ext4 (when the 64bit feature is not enabled), the
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block group descriptor was only 32 bytes long and therefore ends at
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bg_checksum. On an ext4 filesystem with the 64bit feature enabled, the
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block group descriptor expands to at least the 64 bytes described below;
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the size is stored in the superblock.
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If gdt_csum is set and metadata_csum is not set, the block group
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checksum is the crc16 of the FS UUID, the group number, and the group
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descriptor structure. If metadata_csum is set, then the block group
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checksum is the lower 16 bits of the checksum of the FS UUID, the group
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number, and the group descriptor structure. Both block and inode bitmap
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checksums are calculated against the FS UUID, the group number, and the
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entire bitmap.
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The block group descriptor is laid out in ``struct ext4_group_desc``.
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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* - Offset
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- Size
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- Name
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- Description
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* - 0x0
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- __le32
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- bg_block_bitmap_lo
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- Lower 32-bits of location of block bitmap.
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* - 0x4
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- __le32
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- bg_inode_bitmap_lo
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- Lower 32-bits of location of inode bitmap.
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* - 0x8
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- __le32
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- bg_inode_table_lo
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- Lower 32-bits of location of inode table.
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* - 0xC
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- __le16
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- bg_free_blocks_count_lo
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- Lower 16-bits of free block count.
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* - 0xE
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- __le16
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- bg_free_inodes_count_lo
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- Lower 16-bits of free inode count.
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* - 0x10
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- __le16
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- bg_used_dirs_count_lo
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- Lower 16-bits of directory count.
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* - 0x12
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- __le16
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- bg_flags
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- Block group flags. See the bgflags_ table below.
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* - 0x14
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- __le32
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- bg_exclude_bitmap_lo
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- Lower 32-bits of location of snapshot exclusion bitmap.
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* - 0x18
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- __le16
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- bg_block_bitmap_csum_lo
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- Lower 16-bits of the block bitmap checksum.
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* - 0x1A
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- __le16
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- bg_inode_bitmap_csum_lo
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- Lower 16-bits of the inode bitmap checksum.
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* - 0x1C
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- __le16
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- bg_itable_unused_lo
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- Lower 16-bits of unused inode count. If set, we needn't scan past the
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``(sb.s_inodes_per_group - gdt.bg_itable_unused)`` th entry in the
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inode table for this group.
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* - 0x1E
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- __le16
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- bg_checksum
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- Group descriptor checksum; crc16(sb_uuid+group_num+bg_desc) if the
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RO_COMPAT_GDT_CSUM feature is set, or
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crc32c(sb_uuid+group_num+bg_desc) & 0xFFFF if the
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RO_COMPAT_METADATA_CSUM feature is set. The bg_checksum
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field in bg_desc is skipped when calculating crc16 checksum,
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and set to zero if crc32c checksum is used.
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* -
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-
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-
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- These fields only exist if the 64bit feature is enabled and s_desc_size
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> 32.
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* - 0x20
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- __le32
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- bg_block_bitmap_hi
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- Upper 32-bits of location of block bitmap.
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* - 0x24
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- __le32
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- bg_inode_bitmap_hi
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- Upper 32-bits of location of inodes bitmap.
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* - 0x28
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- __le32
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- bg_inode_table_hi
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- Upper 32-bits of location of inodes table.
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* - 0x2C
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- __le16
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- bg_free_blocks_count_hi
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- Upper 16-bits of free block count.
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* - 0x2E
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- __le16
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- bg_free_inodes_count_hi
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- Upper 16-bits of free inode count.
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* - 0x30
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- __le16
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- bg_used_dirs_count_hi
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- Upper 16-bits of directory count.
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* - 0x32
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- __le16
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- bg_itable_unused_hi
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- Upper 16-bits of unused inode count.
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* - 0x34
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- __le32
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- bg_exclude_bitmap_hi
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- Upper 32-bits of location of snapshot exclusion bitmap.
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* - 0x38
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- __le16
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- bg_block_bitmap_csum_hi
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- Upper 16-bits of the block bitmap checksum.
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* - 0x3A
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- __le16
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- bg_inode_bitmap_csum_hi
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- Upper 16-bits of the inode bitmap checksum.
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* - 0x3C
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- __u32
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- bg_reserved
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- Padding to 64 bytes.
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.. _bgflags:
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Block group flags can be any combination of the following:
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.. list-table::
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:widths: 16 64
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:header-rows: 1
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* - Value
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- Description
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* - 0x1
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- inode table and bitmap are not initialized (EXT4_BG_INODE_UNINIT).
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* - 0x2
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- block bitmap is not initialized (EXT4_BG_BLOCK_UNINIT).
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* - 0x4
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- inode table is zeroed (EXT4_BG_INODE_ZEROED).
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